This invention relates to gas chromatography in general and more particularly to an improved sample injection device for process gas chromatography with capillary columns, using a heated evaporation chamber which receives the sample and from which the sample is flushed into the column via a split chamber.
In a known sample injection device, which, however, is not usable with capillary columns, the sample, dissolved in a solvent, is introduced into an evaporation chamber which can be heated. Carrier gas is fed to evaporation chamber which is initially not heated, and to the separation column via a T-shaped tubing branch. A choke is disposed in the feed line to the evaporation chamber. The carrier gas flowing via the choke into the evaporation chamber carriers the solvent away via an outlet opening of the evaporation chamber. After the solvent is evaporated, the outlet opening is closed. The carrier gas is additionally and immediately conducted into the evaporation chamber via a direct path, and the evaporation chamber is heated. In this process, the sample is flushed from the evaporation chamber into separation column via the T-shaped tubing branch. The connection of the evaporation chamber with the carrier gas via the T-shaped tubing branch and the choke is always maintained.
In this sample injection device which is described in U.S. Pat. No. 3,887,345, the solvent is distilled off before the sample is injected, in order to protect the separation column against overloading. In the process, portions of the sample to be analyzed evaporate along with the solvent, so that a reproducible measuring result is not obtained if the composition of the sample changes. Furthermore, the injection process itself takes place over such a long period of time that the injection can be applied only to packed columns.
In comparison to packed columns such as are commonly used in process chromatography, capillary columns have considerably more separating capacity. In a normal column, a time of about 20 minutes is required, for instance, for the complete separation of the components ethyl benzene/o-xylol/m-xylol/p-xylol. If a capillary column is used for this separation problem, the analysis time can be shortened by about a factor of 5 to 10. Chromatographs with such short analysis times are needed for the direct control of plants in process technology.
In order that the capacity of the capillary column can be utilized, the sample injection must be completed in a very short time. The width of many peaks is between 1 and 2 seconds in the case of capillary columns. This means that a time of less than 1 second is available for the sample injection, since the injection time adds to the peak width produced in the separation column. Such short injection times can be achieved only with dosing injectors which work with forced discharge and very small dose volumes (0.1 .mu.l). If the sample is injected with an injector (syringe), a septum is pierced for each dosing operation. The life of such a system is limited to maximally about 200 injections because of the limited durability of the septum.
For sample injection in the case of capillary columns, very small amounts of sample, less than 0.1 .mu.l, must be branched off, since the loading capacity of the capillary columns is extremely small. For obtaining the extremely small amount of sample, the use of a split is known. Thereby, the injected sample is divided up after the evaporation, for instance, in such a manner that of 100 parts of the sample, one part is conducted to the separation column and 99 part are diverted. In the conventional mode of operation, the carrier gas continues to flow during the evaporation process, whereby a segregation of the sample components of different boiling ranges occurs. The high boiling point components flow preferentially along the wall of the evaporation chamber, while the low boiling point ones advance in the middle with higher flow velocity. Since the inlet of the capillary column is also located in the center of the evaporation chamber, the sample composition prevailing there no longer corresponds to the composition of the injected liquid sample.